Discrimination of positions in the acoustic space of barn owls characterized by equal broadband ITD and ILD

This thesis deals with the question, in which way the North American Barn Owl (Tyto furcata pratincola) reacts to sound sources, whose acoustic information is not unambiguously encoded by broadband ITD and broadband ILD. Therefore, in a first experiment impulse responses close to the ear drum of owls were measured at 528 positions. These impulse responses were corrected by a empty measurement, containing the transfer function of the microphones used, to compensate for the influences of the microphones. A fourier-transformation was accomplished to transfer the signal from the time domain into the frequency domain. The resulting head related transfer function was filtered with broadband noise, to create a virtual acoustic space for the following behavioral- and electrophysiological experiments, in which positions with ambiguous broadband ITD and broadband ILD were tested. First the reactions of awake animals to signals originating from such positions were examined in behavioral experiments. In a first step the presentation of the stimuli was accomplished in a free-field environment. Surprisingly no difference in localization was observed between positions with unambiguous acoustic information and such with ambiguous ones. A statistical test showed, that position pairs encoded by ambiguous broadband ITD and broadband ILD can be differentiated by the barn owls. A difference in latencies was not observed. The same results were obtained for experiments in which stimuli were presented in a virtual acoustic space. In a next step, the response of neurons in the optic tectum to those stimuli pairs was recorded in extracellular electrophysiological measurements. As a result of this examination, 42% of the measured cells showed a significantly different response with regard to respective pairs of positions with ambiguous broadband ITD and broadband ILD. this indicates, that there must be other acoustic parameters than broadband ITD and broadband ILD, which have an influence on the response rate of neurons in the optic tectum or the preconnected structures. Additional to the results mentioned above, a full characterization of ten cells was performed, which resulted in the creation of two dimensional maps of neuronal response rates at regarded positions. The maps of neuronal response showed a higher response in the left side of the brain if the broadband ITD was positive and vice versa. Some of the recordings showed, that one position with a combination of ambiguous broadband ITD and broadband ILD is located in an area with higher neuronal response than the other one. Other recordings showed similar response rates for both positions